Control of current spreading in semiconductor laser diodes

ABSTRACT

A semiconductor laser diode and method are described, wherein the path of the current through the device between the positive and negative conductors is controlled. Lateral spread of the gain current in the active region is prevented by implanting protons in areas of the active layer flanking a desired gain region. The implanted regions become less conductive, and prevent lateral spread of the gain current. The position of the implanted regions can be selected so that the gain current only crosses a portion of the active layer that supports desired lateral modes of the laser light.

BACKGROUND OF THE INVENTION

This application is a continuation of U.S. patent application Ser. No.09/710,362, filed Nov. 10, 2000, now U.S. Pat. No. 6,757,313, whichclaims the benefit of U.S. Provisional Application No. 60/164,864, filedNov. 12, 1999. The entire contents of the prior applications areincorporated herein in their entirety by reference. The presentapplication relates to controlling the lateral extent of a region of asemiconductor laser diode that is exposed to a gain current, andspecifically to controlling such lateral extent in a ridge waveguidesemiconductor laser diode adapted to support selected lateral modes ofthe emitted laser light.

SUMMARY OF THE RELATED ART

Typical semiconductor lasers such as laser diodes are formed by a bodyof semiconductor material having a thin, active region formed betweencladding layers and contact regions of opposite polarity. A waveguide isformed in the structure by defining a stripe for light guiding and forcurrent injection. Light is generated in the active region when thestripe region is subject to a current flow between the positive andnegative contact regions. Cladding and confinement regions, amongothers, are placed between the contacts and the active region forguiding and confining the light along the thickness of the layers. Thevarious regions typically are formed as substantially parallel thinlayers grown epitaxially. When the current is greater than the thresholdcurrent for the active waveguide, amplified light is generated. Ingeneral, the greater the current flowing into the active waveguide, themore light is generated.

The active regions can be shaped like a thin layer having a specifiedthickness, length and width. The oscillation of the electric andmagnetic fields of the light waves are restricted to specific modes,depending on the dimensions of the active layer. The longitudinal modeof the light, along the direction of propagation, is determined by thelongitudinal length of the active layer forming the laser cavity.Similarly, the thickness of the active layer restricts oscillation ofthe light waves in the transverse direction, perpendicular to the planeof the layers along which light propagates. By appropriately sizing thethickness of the layer, oscillations can be restricted to thefundamental mode or to other desired modes of the light.

However, in the lateral direction perpendicular to the length of thecavity and in the same plane as the layers, the modes are not limited bythe size of the active layer, but rather by the width of the stripe andof the current flow region. More than one mode can thus co-existsimultaneously within the active layer.

One problem encountered in this type of semiconductor laser diode isthat the light emitted may include more than one optical mode in thelateral direction, as described above. The multi mode light emitted fromthis type of diode is thus of limited use, because it is formed by acomplex pattern of bright and dark areas. Many applications requirelaser light that has a far field pattern consisting of a single brightspot, made, for example, by light that includes only the fundamentalmode. For other uses, a different specific pattern can be required, suchas one that is achieved by generating light having various selectedmodes.

A conventional method used to control the lateral modes of the lightincludes forming a positive conductor on top of the semiconductor layer,having a lateral dimension selected to only support the desired modes.An insulator can be placed between the active layer and the positiveconductor layer outside of that lateral dimension, to prevent currentflowing from the positive conductor outside of the selected region.

This method works for low power applications, but when the gain currentflowing from the positive to the negative conductor and across theactive layer exceeds a certain value, the current tends to spread in thelateral direction as it travels perpendicular to the layers. The degreeof lateral current spreading can also increase with increased gain ordrive current levels. This forms areas of high gain in the active layerthat are larger than what is necessary to support the selected modes.When this occurs, extraneous modes can be supported by these enlargedgain areas of the active layer, and the light emitted is no longer ofonly the desired mode.

Accordingly, there is a need for a device and a method for controllingthe lateral spread of current through an active layer of a semiconductorlaser diode, so that the gain regions of the active layer can be limitedin the lateral direction to only support desired lateral modes of thegenerated laser light, and in particular to support only the fundamentalmode of the laser light.

SUMMARY OF THE INVENTION

The present invention is directed to a semiconductor laser diode and arelated method that is adapted to control the lateral modes of the laserlight generated, so that only desired modes are supported. Inparticular, this result is achieved by controlling the lateral spread ofthe electric current that passes through the active layer, so that onlya selected portion of the active layer has a high gain, resulting inamplification of only the light crossing that portion of the layer. Theother portions of the active layer that flank the selected active regioninhibit the flow of current, and therefore have a lower gain whichresults in less amplification, or no amplification of the light passingthrough those portions. The lateral dimensions of the high gain portionof the active layer can be selected to support only desired modes of thelaser light, such as the fundamental mode or a combination of thefundamental and other modes.

The lateral control of the electric current is achieved by implantinghigh energy ions, such as protons, in the portions of the active layerthat require a reduced conductivity, while shielding from the ionimplant the portion of the active layer where high conductivity,therefore high gain is desired. This shielding can be obtained, forexample, by placing a photoresist layer between the source of ions andthe active layer. The photoresist layer can be shaped with openings thatcorrespond to the size of the desired conductive portion of the activelayer.

To achieve these and other advantages and in accordance with the purposeof the invention as embodied and broadly described, in one aspect theinvention is a ridge waveguide semiconductor laser diode adapted tosupport desired lateral modes of generated light, comprising a firstconductor layer for application of a current, a second conductor layerfacing the first conductor layer, an active layer disposed between thefirst and second conductor layers, a conduction region of the activelayer adapted for conducting the current, and reduced conductivityregions of the active layer, flanking the conduction region, adapted toimpede passage of the current.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the objects, advantagesand principles of the invention.

In the drawings:

FIG. 1 is a schematic perspective view of one embodiment of asemiconductor laser diode incorporating the present invention;

FIG. 2 is a schematic perspective view of the semiconductor laser diodeshown in FIG. 1, also including a barrier layer;

FIG. 3 is a schematic front elevation view showing a detail of theembodiment of FIG. 1; and

FIG. 4 is a schematic view showing individual semiconductor laserelements disposed in an array.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Semiconductor laser diodes are used in a variety of devices such asoptical data storage and compact disc drives, for printing processessuch as those used in laser printers, and also for displays. For certainapplications, a plurality of laser diodes can be assembled in an array,so that the light from all of the arrayed laser diodes has the same modeand in some cases also the same phase.

For high brightness applications, a gain current up to 20 or 30 timesthe threshold current of the active layer in the laser diodes can beused to obtain a high brightness spot or beam from the semiconductormaterial.

FIG. 1 shows one embodiment of a semiconductor laser diode according tothe present invention. Semiconductor lasers consist of epitaxial layersgrown on a single-crystal substrate 15. Typically, the substrate 15 isn-type. The exemplary semiconductor laser diode 1 has an active layer 10that can include a quantum well structure. Active layer 10 can beformed, for example, of un-doped InGaAs or InGaAsP. A positive conductor12 can be applied facing one surface of the active layer 10, and severalp-type clad region layers and confinement layers can also be depositedin region 18, between the active layer 10 and the positive conductor 12,according to a conventional manner of construction of semiconductorlaser devices. A negative conductor layer 14 can be formed on thesubstrate 15, facing the opposite surface of active layer 10. Aconventional arrangement of confinement layers and n-type clad layerscan also be disposed in region 20, between the active layer 10 and thesubstrate 15.

Positive conductor layer 12 can be a strip as shown in FIG. 1, or canextend the entire width of semiconductor laser diode 1. The strip shapedpositive conductor layer 12 can preferably have dimensions correspondingto a region of active layer 10 that supports the desired optical modes.A dielectric insulator layer 16 can be used to prevent the flow ofcurrent from entering the top layers of region 18, outside of a selectedlateral area. Insulator layer 16 thus defines an opening 17 in theinsulator layer, that corresponds to the area where positive conductor12 is in contact with region 18. In this manner, the current flowingfrom positive conductor 12 to negative conductor 14 is allowed to enterthe portion of region 18 under opening 17, but is prevented fromentering the remainder of region 18 by insulator layer 16. In adifferent embodiment, insulator 16 can be omitted from the device,particularly if positive conductor 12 is shaped as a strip of desiredwidth, and does not extend the entire width of semiconductor laser diode1.

The construction of semiconductor laser diode 1 can also include aridged waveguide 22 extending parallel to a longitudinal axis ofsemiconductor laser device 1, and extending along the entire length L ofthe device. For example, ridge waveguide 22 can have a width of about3-5 microns. Ridge waveguide 22 channels the light being emitted andamplified in active layer 10, so that the light is directed for the mostpart along the longitudinal dimension of the semiconductor laser device1.

In the lateral direction, along the active layer 10, the extent of thelaser light is governed by the width of the waveguide, the lateral indexstep between the waveguide and the region external to it, and the amountof lateral current spread. In the case of a ridge waveguide, the lateralindex step is the index difference between the region under the ridge22, and the regions under the channels 23 on each side of the ridge 22.

If the index step is not present, the waveguide is “gain guided”. Inthis case, the light is guided along the current path by virtue of theabsorption difference, or gain vs. loss ratio, between the current flowregion and its lateral regions. When the index step is present, thewaveguide is “index guided”, so that guiding of the light is achieved bythe index step. Index guiding can be more advantageous than gainguiding, because it results in single-spatial mode operation and reducedastigmatism.

At low power settings, only a small portion of the active layer 10,approximately corresponding to the width of the non-insulated region ofpositive conductor layer 12, is crossed by the current. At high powersettings, when the current flowing from positive conductor 12 tonegative conductor 14 is relatively large, the combination of astripe-like positive conductor 12 and an insulator 16 is insufficient tomaintain the flow of current only within the general dimension ofopening 17. Instead, the current tends to spread laterally outward to aregion of active layer 10 having greater width than the opening 17. Thecurrent thus no longer follows a straight line from positive conductor12 to negative conductor 14, but instead flares laterally outwards, asschematically shown by the dashed line 31 in FIG. 1. This broadens theeffective width of the gain region generating light, and allows thewaveguide to support additional lateral modes.

To prevent the occurrence of unwanted modes, active layer 10 is dividedin a defined gain region 24 of high conductivity, through which thecurrent flowing from positive conductor element 12 to negative conductorelement 14 can easily pass, and flanking regions 26 of reducedconductivity, through which the current cannot easily pass. In thismanner, the current flowing from positive conductor element 12 tonegative element 14 is prevented from spreading laterally away fromdefined gain region 24, and follows a path shown by the solid line 32.The lateral dimension of the high gain portion of active layer 10 isthus limited to the size of defined gain region 24. By properly sizingthe defined gain region 24, only the desired lateral modes of the laserlight can be sustained, while other modes are not amplified and willdecay.

For example, defined gain region 24 could be sized to only support thefundamental mode of the laser light in active layer 10, so that a sharp,single spot output laser beam can be generated by the device. Generationof laser light of the fundamental mode is useful for lasers used intelecommunications. For other applications, different modes can beuseful. For example, a non-gaussian beam formed by the fundamental andthe second transverse mode can be useful in laser printing applicationsto improve printing sharpness.

The reduced conductivity regions 26 formed in the active layer 10 can beobtained, for example, by implanting ions such as protons in thematerial of active layer 10. The proton implant damages the structure ofthe active layer 10, and causes the affected region of active layer 10to become non-conducting. The extent of the transformation incurred byactive layer 10 is dependent on the strength and the duration of theproton implant. For example, successful results can be obtained byimplanting protons having an energy of between approximately 130 KeV and170 KeV. The implant can preferably have a duration of between about 1and 5 minutes, and can be repeated more than once.

As shown in FIG. 1, the laser light is amplified within a lightamplification portion 30 of defined gain region 24. The actual shape oflight amplification portion 30 depends on the desired mode beingsustained. For example, in the case shown, the fundamental mode issupported, which results in a light amplification portion 30 shaped likea single elliptical spot through active layer 10, where the lightamplification takes place.

The location within active layer 10 of reduced conductivity regions 26must be selected carefully. The width of the defined gain region 24 ofactive layer 10 determines which lateral modes of the laser light willbe supported, therefore reduced conductivity regions 26 must be placedsufficiently close to light amplification portion 30 of active layer 10,so that additional modes will not be sustained by the active layer 10.

To satisfy these requirements, interface 40 between defined gain region24 and reduced conductivity regions 26 must be selected to be justoutside light amplification portion 30 of active layer 10. At the sametime, interface 40 must not be so far from light amplification portion30 that the defined gain region 24 can support modes other than thedesired mode of the laser light.

The manufacturing process for semiconductor laser diode 1 will bedescribed with reference to FIG. 2. Positive and negative conductorlayers 12 and 14, insulating layer 16, active layer 10 and the remaininglayers forming semiconductor laser diode 1 are grown in a conventionalmanner. In a preferred embodiment of the device according to the presentinvention, reduced conductivity areas 26 are formed by implantingprotons in the layers of the device. A source of protons 44 can be, forexample, hydrogen.

A barrier layer such as photoresist layer 42 is used to shield from theprotons areas of active layer 10 that are to remain conductive. Forexample, photoresist layer 42 can be placed above defined gain region24, so that protons generated by source 44 are stopped by photoresistlayer 42, and do not affect defined gain region 24. The remainingportions of the device, above and including the reduced conductivityregions 26, are implanted with protons, resulting in a loss ofconductivity for those regions. In an embodiment according to theinvention, only sections lying below the channels 23 on the sides of theridge 22 are implanted. The depth of the implant can correspondapproximately to the thickness of clad region 18 and can reach throughactive layer 10.

FIG. 3 shows a detail of a region of the semiconductor laser depicted inFIG. 1. This exemplary embodiment includes an implant region of constantdepth ‘d’ obtained, for example, with an implant of given energy andduration. Because of the presence of channels 23 on the sides of ridge22, the implant depth ‘d’ reaches active layer 10 in regions 26, belowchannels 23. Regions 26′, located beyond channels 23, also receive animplant of depth ‘d’. However, the layers of the laser diode above theactive layer 10 have greater thickness at that point, and thus theimplant in region 26′ may not reach the active layer 10.

FIG. 3 also shows lines 32 that indicate the path of current travelingthrough active layer 10 from positive conductor 12, and lines 33indicating the path of spreading current blocked by reduced conductivityareas 26.

Protons from source 44 can travel across metallic layers and the variouslayers that form semiconductor laser diode 1, so that the proton implantcan take place in a single step process, after all the layers of thewafer have been grown in a conventional manner. Photoresist layer 42 canbe made, for example, of a long chain polymer such as SHIPLEYPhotoresist Type AZ 4620, having a thickness of about 5 μm to 7 μm.However, other materials that absorb protons from source 44 can be usedin photoresist layer 42.

Since the energy of the implant determines the amount of loss occurringin the implanted layer, the loss in selected regions of the active layer10 can be controlled with the location and energy of the implant. Inthis manner, the waveguide loss in the lateral direction can be selectedby modifying the location of the implant. This loss introduced by theimplant is another mechanism that can be used to control the laser lightmode in the waveguide, in addition to controlling the current spread inreduced conductivity areas 26.

In a preferred embodiment, photoresist layer 42 can be placed above theouter surface of conductor layer 12, and can be removed after theimplant has been performed. However, other configurations of photoresistlayer 42 can be utilized, as long as photoresist layer 42 is placedbetween the source of protons 44 and the areas that are to remainconductive after the implant.

The semiconductor laser diode according to the invention is well suitedfor use in an array of laser diodes. For example, FIG. 4 shows anexemplary array of individually-addressable laser elements 50. Eachelement 50 includes a ridge single-mode waveguide element 52, shownupside down in the figure. In an exemplary embodiment, a dielectriclayer 54 and a p-side metallization layer 56 are formed with anappropriate shape and thickness to define the ridges 58. The elements 50are thermally and electrically separated by V-grooves 64, etched throughthe active region 60. In an exemplary embodiment, the element 50 areseparated by approximately 50 μm, resulting in an array that containsmore than 200 laser elements, with a density of 50 elements per cm. Allthe laser elements in the exemplary embodiment produce a spot of laserlight 62 in active layer 60 having the same mode characteristics.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the structure and themethodology of the present invention, without departing from the spiritor scope of the invention. Thus, the present invention is intended toencompass the modifications and variations that come within the scope ofthe appended claims and their equivalents.

1. A method of forming a ridge waveguide semiconductor laser diode, themethod comprising: forming a p-doped layer, an active layer, and an-doped layer above a substrate, at least one of the layers defining aridge waveguide having a first width at a bottom of the ridge; disposinga barrier layer above the doped layers and above the active layer,wherein the barrier layer forms a mask defining an opening having asecond width; and implanting high energy ions though the barrier layerinto the active layer to form reduced conductivity regions within theactive layer that flank a defined gain region within the active layer,wherein the defined gain region has a third width greater than the firstwidth and wherein the third width is selected such that the defined gainregion supports a fundamental lateral mode of the light, while higherorder modes are not supported due to their overlap with the reducedconductivity regions.
 2. The method of claim 1, wherein disposing thebarrier layer comprises disposing a photoresist layer.
 3. The method ofclaim 1, wherein implanting high energy ions comprises implantingprotons having an energy between about 130 keV and about 170 keV.
 4. Themethod of claim 1, wherein implanting high energy ions comprisesimplanting protons for a time period of at least one minute.
 5. Themethod of claim 1, wherein implanting high energy ions comprisesimplanting protons for a time period of at least five minutes.
 6. Themethod of claim 1, wherein the defined gain region has a first loss togenerated light and the reduced conductivity regions have a second lossto generated light greater than the first loss.
 7. The method of claim1, wherein the third width is selected such that the reducedconductivity regions flanking the defined gain region introducesignificant loss to generated light in a higher-order mode, but do notintroduce significant loss to generated light in a fundamental mode. 8.The method of claim 1, wherein the active layer is fonned of a least oneof GaAs, InGaAs, AlInGaAs and InGaAsP.
 9. The method of claim 1, whereinthe reduced conductivity regions have a first index of refraction andthe defined gain region has a second index of refraction greater thanthe first index of refraction.
 10. The method of claim 1, furthercomprising disposing an insulator layer over one of the doped layersoutside the ridge.
 11. The method of claim 1, wherein the first width isabout 3 to 5 microns.